[rss] Overhaul of RSS module

I found the RSS module to be pretty hard to use, so I've updated it in the following ways:

- Added clearer messages and docstrings
- Improved validation of command arguments
- Set the read_feeds function to run using the @interval decorator
- Using a more appropriate URL value from feedparser
- Added support for updating existing feeds

Issues:
- Currently the interval value is hard-coded at 20 seconds, since the interval decorator doesn't seem to support changing the value or stopping/starting the function. I'd like to try and fix that if possible; suggestions would be welcome.